1. Field of the Invention
The present invention relates to an optical pickup in an optical recording and reproducing apparatus.
2. Description of Related Art
Optical recording and reproducing apparatuses include an optical disk player which reads recorded information from an optical recording medium or an optical disk, such as a so-called LD (Laser Disc), CD (Compact Disc) or DVD (Digital Video Disc). There also is a compatible disk player which reads information from those different kinds of optical disks.
An optical pickup of that compatible disk player like an ordinary one has an optical system which irradiates a light beam to an optical disk and reads return light from the optical disk.
Those optical disks as optical information recording media are designed with different specifications including the numerical aperture NA, the thickness of the substrate and the optimal wavelength of read light. Implementation of an optical pickup for a compatible player for LD, CD and DVD therefore requires that at least two differences in the numerical aperture NA and substrate thickness should be compensated.
For example, a two-focus pickup using a holographic lens (disclosed in Japanese Patent No. 2532818 and Japanese Patent Kokai No. Hei 7-98431) has a composite objective lens, which includes a convex objective lens and a holographic lens, and a diffraction grating with concentric ring-shaped recesses and projections, i.e., diffraction grooves, provided on a transparent plate of the holographic lens, and the performance of a concave lens is imparted on this transparent plate to form a focal point on the recording surface in accordance with each optical disk. At this time, the light beam directly passes through the area where the diffraction grooves are not formed, and converges, together with the zero-order diffraction light, onto the objective lens, resulting in differences in numerical aperture between the transmitted an light and the zero-order diffraction light and the first-order diffraction light. The first-order diffraction light that has been diffracted by the diffraction grooves is used to read information from a CD which has a small numerical aperture, and the transmitted light and the zero-order diffraction light which have larger numerical apertures are used to read information from a DVD.
This conventional compatible player is designed to form read spots by means of a single, common light source. Generally, a light source which launches read light having a wavelength of 650 nm suitable for reproduction of a DVD is also used to play back a CD. To play back a CD-R (CD Recordable or R-CD (Recordable CD)), which can be written once by a light source with a wavelength of 780 nm, by using this read light, therefore, satisfactory reproduction signals cannot be acquired due to the insufficient sensitivity that results from a difference in wavelength.
To realize a compatible player capable of adequately recording and reproducing information on, and from, a CD-R as well as an LD, CD and DVD, it is essential to cope with at least three differences in numerical aperture NA, substrate thickness and the wavelength of the light source in use (780-nm type and 650-nm type). To implement a compatible player for an LD, CD, DVD and CD-R, therefore, it is. necessary to design an optical pickup or head using a light source of multiple wavelengths suitable for the respective disks, not a light source of a single wavelength. Constructing an optical system like a prism or lens using a plurality of light sources, however, complicates and enlarges the whole optical pickup or head.